The process according to the invention is based on the principle of reacting reactive systems which contain isocyanate groups and oxazolidine groups with water so that the chain lengthening reaction is accompanied mainly by the formation of urea groups from the isocyanate groups and from the amino groups which are liberated from the oxazolidines hydrolytically, while the comparatively inert hydroxyl groups which are also liberated from the oxazolidine groups by hydrolytic splitting do not take part in the reaction but are left in side chains in the products of the process.
The principle of cross-linking reactive systems which contain isocyanate groups and oxazolidine groups by the effect of moisture has already been disclosed in numerous publications, for example German Offenlegungsschriften Nos. 1,952,091; 1,952,092; 2,018,233; 2,446,438 and 2,458,588 and U.S. Patent Nos. 3,661,923; 3,743,626; 3,864,335 and 4,002,601. The compositions described in these publications are generally systems containing oxazolidines and isocyanates which are stable in storage in the absence of moisture and which react under the influence of atmospheric moisture, i.e. in the presence of traces of water, to form high molecular weight cross-linked structures. Due to the slow access of water in the form of atmospheric moisture, both the amino groups and the hydroxyl groups of the hydrolytically split oxazolidine react with the isocyanate groups. The polyisocyanate component, therefore, undergoes not so much chain lengthening reactions but rather true cross-linking reactions to give rise to products in which there are virtually no more hydroxyl groups in side-chains. No indications of the process according to the invention described below can be found in the aforesaid publications.